1. Field of Technology
This disclosure relates to the mitigation of glare from reflective surfaces, for example, mirrored windows in a building.
2. Description of the Related Art
Partially reflective window coatings (e.g., thin films of sputtered metal) and films (e.g., metal-impregnated and Bragg mirror polymer films) have been widely used for decades as a method for reducing solar heat gain and thus improving the energy performance of buildings. The design, composition, and benefits of reflective coatings are well documented and need no further elaboration here, except to note that the reduction in solar heat gain, while clearly beneficial in hot, sunny conditions, may be detrimental in cold weather as it interferes with passive solar heating.
Switchable mirrors also exist which are based on reversible metal hydride and metal lithide chemistry described, for example, in U.S. Pat. No. 7,042,615 to Richardson. These switchable mirrors, which are analogous to rechargeable batteries, may rely on the physical migration of ions across a barrier under the influence of an electric field and, therefore, have limited switching speeds and cycle lifetimes. In addition, electrically operated “light valves” that combine liquid crystals with one or more reflective polarizers are described, for example, in U.S. Pat. No. 6,486,997 to Bruzzone et al. In these devices, a liquid crystal typically serves as an electrotropic depolarizer, i.e., a means of variably altering or rotating the polarity of the light that passes through it under the influence of an electric field. Some of these devices can be thought of as switchable mirrors, although they are rarely described that way, since their primary application is in video displays and advanced optics.
Switchable electric light valves which do not require polarizers but are diffusive forward scatterers or diffusive reflectors also exist. This is because liquid crystals may in fact be Bragg reflectors with different reflection bands in these applications, with a reflective, diffusive, or forward-scattering mode, and a more transmissive mode. These include the polymer-dispersed liquid crystal (PDLC) display, the cholesteric liquid crystal display (Ch-LCD), the Halmeier display, and the Guest-Host display. The PDLC is an electrochromic device where the index of refraction of liquid crystal droplets embedded in another material is changed electrically, resulting in more scattering of the light in one mode than another. The Ch-LCD has two stable states, the reflective planar and focal conic texture. The reflective planar structure reflects light if the Bragg reflection condition is met and thus acts as a Bragg reflector for one circular polarization of light, while the reflective focal conic transmits more of the light. The Guest-host display utilizes dyes dispersed in a liquid crystal, which absorb more light when in one orientation than in another. The orientation of the dyes is dependent on the orientation of the liquid crystal, which can be determined using an electric voltage. Polymer stabilized liquid crystals are created when polymers and liquid crystals are mixed and photopolymerized together to among other things establish the orientation of the liquid crystals.
In addition, U.S. Pat. No. 6,288,840 to Perkins et al., discloses a type of reflective polarizer called a “wire grid polarizer” which consists of a nanoscale array of closely spaced, parallel metal wires on a transparent substrate, such that light of one linear polarity which strikes the wires is reflected while light of opposite linear polarity is transmitted through the substrate. Wire grid polarizers may be a component of some reflective and thermoreflective optical filters. It is additionally possible to construct polarizers using distributed Bragg reflector technology, such as 3M's Dual Brightness Enhancement Film (DBEF). In these polarizers, some layers have different optical indices in one transverse direction than in another, creating a polarizer.
Any surface which presents a mirror finish in the presence of a light source, whether indoors or outdoors, has the potential to create glare, i.e., a condition in which background illumination approaches, equals, or exceeds the illumination of objects in the foreground, which in some case can lead to discomfort or reduced visibility. For this reason, mirrored films are banned or discouraged in some jurisdictions and are the subject of increased scrutiny in others.
Antireflection coatings are widely used to reduce glare from bright light sources on transparent optics such as eyeglass and binocular lenses. Reflection from a transparent surface occurs because the index of refraction of the transparent material does not match that of the surrounding medium (e.g., air, water, or vacuum). The greater the mismatch, the greater the reflection. A standard antireflection coating has an index of refraction approximately halfway between that of the transparent material and the surrounding medium. More sophisticated, nanostructured coatings (e.g., arrays of vertically oriented nanoscale cones) may present an outer surface which is mostly air and thus has an index of refraction close to air, and an inner surface which is mostly solid and has an index of refraction essentially identical to the transparent material on which it sits. In this case, reflection may be reduced virtually to zero.
However the high reflectivity of a polished metal surface, dielectric mirror, distributed Bragg reflector, or other mirror does not rely on an index of refraction mismatch with the surrounding air. Instead, reflection is achieved by photon interaction with conduction electrons (e.g., in a metal) or by constructive and destructive interference between layers of different material (e.g., in a dielectric mirror). Thus, a transparent antireflection coating will have little effect on the reflectivity of such mirrors. However, a diffusive coating will cause the reflected light to be scattered, producing a white surface which may still be highly reflective, but not specular (i.e., mirror-like) and therefore not capable of reflecting or transmitting an intelligible, collimated image.
In addition, there are numerous examples of “metamaterials” or nanostructured materials or devices which interfere with light waves in such a way that the material appears to have a negative index of refraction and thus to violate many of the presumed “laws” of classical optics. The scientific paper “Asymmetric Propagation of Electromagnetic Waves through a Planar Chiral Structure” (V. A. Fedotov, P. L. Mladyonov, S. L. Prosvirnin, A. V. Rogacheva, Y. Chen, and N. I. Zheludev, Physical Review Letters 97, 167401, 17 Oct. 2006) discloses a chiral, planar structure consisting of “fish scale” patterns of aluminum nanowire on a transparent substrate. In essence, the structure is a type of wire grid polarizer, although one that reflects and transmits circularly polarized light rather than linearly polarized light. Because its chiral nature is different depending on which surface of the polarizer is being observed, for light of mixed, random polarity (e.g., sunlight), this structure has an additional property of being asymmetrically transmissive, i.e., it is more transmissive to light passing through it in one direction than in the other.
In addition, there are numerous varieties of optical diffusers, including etched glass and polymer films, which partially randomize the direction of photons passing through them, while exhibiting modest reflection and very low absorption. In the case of “forward scattering” diffusers which affect the direction of most incident light by substantially less than 90 degrees, as much as 80% of the light striking the diffuser is passed through, with less than 20% being absorbed or reflected back. Diffusers are commonly employed in privacy windows, skylights, video displays, and electric lighting.
There are also so-called “one-way mirrors” which are commonly used as privacy windows, particularly in building interiors (e.g., to separate a supervisor's office from the workers being supervised). However, these are not true one-way devices. Rather, they are simply partially mirrored transparent glass, equally reflective in both directions, and the “one-way” effect requires that the area on one side of the glass be more brightly illuminated than the area on the other side. If these lighting conditions are reversed, then the privacy effect is reversed as well (e.g., the supervisor may see his own reflection, whereas the employees may have a clear view of the supervisor).
Finally, various types of prismatic films use etched structures or variable index of refraction structures to bend the light passing through them. Examples include Fresnel concentrating lenses and polymer “privacy filters” that can be applied to laptop screens or video displays to narrow or widen the viewing angle.
The information included in this Background section of the specification, including any references cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded as subject matter by which the scope of the invention is to be bound.